Color-switching nano-laser brings real optical nanotech much closer

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Lasers have revolutionized the world in many ways — they allow everything from satellite communication to precision-cut diamond — but even so, they were supposed to be so much more. From missile defense to in-home cooking, even now science is only beginning to realize the true potential of lasers. Traditionally, most of our biggest laser project have been, well, big, scaling up existing lasers and achieving some very real success. But in modern science, it’s the benefits of small lasers that currently demand the most attention. This week, a team from Northwestern University announced what could be a major step forward in the miniaturization of lasers, as it has created the first working nano-laser that can change color at will.

To understand why that’s a significant achievement, it’s important to understand how nano-lasers generally work. Traditional nano-lasers (like the one invented by this very team from Northwestern) use a solid “gain” medium to turn input energy (usually in the form of electricity) into output energy (in the form of photons). Making this gain component out of different materials with different physical properties will result in output photons with a different wavelength; in general, if you want a different color of laser, you simply swap out the gain medium.

Gold nanoparticles, surrounded by the appropriate gain liquid, can concentrate and amplify the light.

For tiny lasers integrated into a complex device, though, swapping out a solid piece of material simply is not an option. That’s why this team decided to try using a liquid gain medium — one that could be swapped out within a micro-device simply by flowing it through sealed channels. This allows real-time tuning of the laser’s wavelength through control of the flow of these lasing liquids.

Want to switch from a red to a green laser? Simply apply some specific pressure to the chip’s plumbing such that the red medium is pushed to displace the green medium in the amplification chamber. This chamber contains specially placed gold nanoparticles that interact with the gain liquids to amplify the input signal to create powerful, highly specific output wavelengths. This makes it not only the first liquid nano-laser, but the first nano-laser of any type that can have its wavelength tuned in real time.

Artist’s rendering of a prototype hybrid-optical processor from IBM.

That’s important because wavelength is the primary tribute that dictates how a photon interacts with whatever it touches. That’s why different wavelengths of radiation are good for different sorts of experiments — high-wavelength radiation like radio waves can be great for collecting data about other galaxies, for instance, but useless for looking at tiny objects like cells or even atoms. An fixed-wavelength optical device could only “see” in that very specific part of the spectrum, whereas a tunable laser could switch “vision modes” like a Predator, collecting different sorts of data at will.

In that spirit, one of the most talked-about possible applications for nano-lasers is in diagnostic medicine — your GP takes a blood sample and uses a nano-laser device to quickly check it for various possible contaminants. Doing such a wide-ranging analysis will require the use of a wide range of wavelengths, though, and it’s difficult enough to create one nano-laser of just a single type. A tunable nano-laser could bring nano-optics much closer to reality by allowing a single laser source to cycle through the various interrogating wavelengths necessary for medical use.

Beyond medicine, nano-lasers could also allow super-fast optical processors with the ability to continue Moore’s Law for some time to come. They could be used to create measurement devices so small they can be suspended in a solution and injected underground for mapping of hard-to-reach areas — or perhaps injected into the bloodstream of a human being.

Nano-lasers have already been made as small as a virus. Now, thanks to this team from Northwestern, such a nano-scale device could actually be many different devices in one. It could bring real-world nano-optical devices a whole lot closer to reality.

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